1. Field of the Invention
The present invention relates to an image processing apparatus, which performs multi-level conversion processing of a document image including a photograph image using an error diffusion method to satisfy faithful gradation reproducibility, and maintains good image quality by minimizing a variation in gradation characteristics under the influence of a change in environment, aging, and the like.
2. Description of the Related Art
In association with an image forming apparatus for printing, copying, or displaying a document original including a photograph image or image data obtained via document creation processing and image edit processing using a computer, an error diffusion method has been proposed as a binary conversion method which can satisfy gradation characteristics of both a photograph image area and a character/line image area, and can provide higher resolution characteristics of the character/line image area than those of an ordered dither method. The error diffusion method is disclosed in R. W. Floyd L. Steinberg: Pro: SID Vol. 17(2), pp 75-77, 1976. In the error diffusion method, a level obtained by multiplying multi level (binarized) errors of already multi level (binarized) pixels around a pixel of interest with a weighting coefficient is added to the pixel of interest, the sum is multi-level (binary) converted by a fixed threshold level, and a multi level (binarized) error generated at that time is distributed to the surrounding pixels. More specifically, in this method, although an output level corresponding to each pixel unit is predetermined multi-level data (binary data), the sum total of errors associated with a local area of an output image corresponding to that of an original image (multi-level image data) having continuous gradation is eliminated or minimized, thus compensating for gradation characteristics. For this reason, input gradation information can be expressed by the number of output levels smaller than that of the gradation levels of input pixel signals.
In such an error diffusion processing means, a multi level signal means a predetermined number of levels such as the number of write levels of an image forming system. In contrast to this, an input signal has a physically different dimension from that of a multi level signal to be output since it is a signal read by, e.g., a scanner or an image created using a computer, and means reflection factor or density information. For this reason, in error calculations, a contradiction upon calculations, i.e., addition/subtractions between different dimensions, occurs.
When the gradation characteristics of an image output from an image forming apparatus for forming an image based on a multi level signal as a result of error diffusion processing vary due to a change in environment, aging, or the like, a gamma correction means or color correction means, a multi level signal conversion means, or the like is externally arranged at the upstream or downstream side of an error diffusion processing unit, so as to calculate correction characteristics in correspondence with a detection result of a variation in gradation characteristics, and to modify, e.g., table contents. However, even in an input/output relationship free from the above-mentioned contradiction upon calculations between different dimensions when no variation in gradation characteristics occurs, if the gradation characteristics of an output image vary, and the gamma table is corrected, the relationship between the corrected input pixel signal and the multi level undesirably changes, thus deteriorating accuracy of error calculations.
Furthermore, in error diffusion processing for performing multi-level conversion using a fixed threshold level, a macroscopic change in gradation characteristics can be eliminated upon practical variations in gradation characteristics and the number of gradation levels corresponding to levels of multi level signals, but deterioration of image quality such as a decrease in resolution, conspicuousness of a texture, and the like cannot be suppressed.